The present invention relates to data storage and, more particularly, to a method of archiving data in accordance with an expected lifetime of the data.
FIGS. 1 and 2 illustrates the storage of a bit, either a zero bit or a one bit, in a cell of an electronically programmable memory (EPROM) such as a flash memory. For historical reasons, this process of storing data in a EPROM is called “programming” the EPROM. Nominally, a zero bit is represented by a cell threshold voltage V0 and a one bit is represented by a cell threshold voltage V1. Initially, the cell has a nominal threshold voltage V1. For example, after a block of a flash memory has been erased, all the cells have nominal threshold voltages V1. Because of unavoidable inaccuracies in the initializations of the cells, the actual threshold voltages are distributed around the nominal threshold voltage V1 according to a distribution curve 10. Then, to each cell that is to store a zero bit, a train 12 of programming voltage pulses 14 is applied, in order to inject electrons from the cell's silicon substrate through the cell's oxide layer into the cell's floating gate. Because the electrons move through the oxide layer by quantum mechanical tunneling or by hot injection, and because of non-uniformities in the cells' structures, the voltage required to inject enough electrons to increase the threshold voltage from V1 to V0 cannot be predicted accurately in advance. The voltage of the first pulse 14 is a starting voltage Vs+a programming voltage increment ΔV. Every subsequent pulse 14 is higher than its predecessor by ΔV. After each pulse 14 is applied, the cell is tested to see if its threshold voltage is sufficiently close to V0. If the threshold voltage is sufficiently close to V0 then the programming of cell is complete. Otherwise, the next pulse 14 is applied to the cell and the threshold voltage of the cell again is tested. Because the initial threshold voltages are distributed about the nominal voltage V1, and because of inaccuracies in the programming, the threshold voltages of the cells that store zero bits also are distributed about the nominal threshold voltage V0, according to a distribution curve 16.
Data are read from the EPROM cells by sensing the cells' threshold voltages. A threshold voltage greater than a transition threshold voltage VT halfway between V0 and V1 is interpreted as a zero bit. A threshold voltage less than VT is interpreted as a one bit. Over time, primarily because of the tunneling of electrons from the floating gates back to the substrate, the distributions 10 and 16 tend to become broader. The difference between threshold voltages V0 and V1 is selected to be great enough so that, over the lifetime of the EPROM, the likelihoods that the lower end 18 of distribution 16 will descend below VT and that the upper end 20 of distribution 10 will ascend above VT are negligible.